This invention relates to improvements in three-phase permanent magnet rotary electrical machines such as motors and generators. More particularly, the invention relates to such improvements which minimize reluctance torque and electromagnetic torque ripple while maximizing compactness, energy efficiency and motor starting torque per unit volume of winding.
Permanent magnet motors having slotted armatures and multi-coil phases have been produced in the past utilizing an odd number of slots and armature poles and an even number of permanent magnet poles to reduce reluctance torque and thus vibration, as exemplified by the motors shown in Ban U.S. Pat. No. 4,437,029 and Aoki U.S. Pat. No. 4,532,449. However, the coils of the windings for such motors are either superimposed upon each other or, if not superimposed, require the use of more than three phases. In the former case, the superimposed coils tend to maximize the amount of wire in the winding, thereby maximizing both its volume and impedance and minimizing its efficiency and torque (or emf) per turn. In the latter case, the large number of phases is undesirable due to the need for a correspondingly high number of phase-switching circuits which add complexity and expense.
Three-phase permanent magnet motors, having multi-coil phases wherein the individual coils are not superimposed upon each other, have been designed as shown in FIGS. 2, 2A and 2B herein. However, even though the coils do not overlap, the phases do overlap since the coils of one phase are interstitially inserted between the coils of another phase. Such winding configuration, although minimizing self-inductance which is beneficial in high-speed applications, produces electromagnetic torque ripple and reduced starting torque per unit volume of wire, both of which are disadvantageous in many applications.
Conversely, motors having equal numbers of armature slots and permanent magnet poles, as exemplified by Hahn U.S. Pat. No. 4,188,556, are characterized by considerable reluctance, or "cogging", torque which produces harmful vibration in many applications.
Accordingly, a need exists for a three-phase, permanent magnet rotary electrical machine which compatibly satisfies all of the objectives of compactness, minimal reluctance torque and electromagnetic torque ripple, and maximum energy efficiency and starting torque per unit volume of wire.